The determination of the visual range in a turbid medium, i.e. the determination of a range, outside of which two separate or different-colored objects are no longer reliably distinguishable to an observer, is a technical problem that is particularly relevant to the control of vehicles, since the visual range substantially determines the maximum safe speed that may be traveled. The multitude of accidents occurring per year under conditions of limited sight distance, e.g. in the case of fog, indicates that it is very difficult for a human driver to quantitatively determine the current visual range and adjust the speed of his or her vehicle to this visual range.
Therefore, it is desirable to be provided with methods for measuring the visual range, which can be used, for example, in future driver assistance systems, in order to supply, in each case, a warning signal in response to a maximum speed set as a function of the measured visual range being exceeded, and thus to cause the driver to reduce his speed. Quantitative determination of the visual range is also of particular importance in future driver assistance systems that take away parts of the driving tasks of the driver, such as the longitudinal vehicle control system (automatic gasoline/brake intervention in an adaptive driving-speed control system.
Two different types of visual ranges are of principle interest. In this context, the first is the objective visual range (standard visual range), which is given by the attenuation of an optical signal in response to penetrating a certain segment of the medium in which the visual range is to be measured.
Secondly, one would like to determine the subjective visual range of the driver. This is influenced by factors such as external lighting, contrasts, surroundings, the size of surrounding objects, as well as the individual visual acuity of the driver. When driving, for example, at night in nonilluminated surroundings, the subjective visual range of the driver is therefore limited to the boundary of the vehicle-headlight cones. But if one is driving at night on an illuminated avenue, then the visual range is considerably larger. In both cases, the attenuation of a light signal in the atmosphere, and thus the standard visual range, are the same. Due to the multitude of possible environmental effects and individual parameters, it is considerably more difficult to determine the subjective visual range than the objective visual range, i.e. it is only possible to a limited extent.
Techniques for measuring the standard visual range are being tested for use in motor vehicles. These are essentially active methods, which operate according to the backscattering principle. In this context, an optical signal is emitted into the medium to be measured. The optical signal scattered back is picked up by an appropriate sensor, and the transmittance of the medium with respect to the emitted signal is deduced by analyzing the time characteristic of the received signal. In order to prevent the driver from being disturbed by the measurement during use in a motor vehicle, an optical signal in the near-infrared range must be used.
Since active methods generally work with their own light source, they are limited to the determination of the standard visual range. The lighting conditions actually perceived by the driver cannot be taken into consideration. Since the subjective visual range perceived by the driver cannot be larger than the standard visual range, but is often considerably smaller, the suitability of these methods for use in driver-assistance systems is very limited.
Besides the active methods mentioned, passive methods, which only work with the light already present in the environment, are also proposed. An example of such a passive method and a device for implementing it is referred to in EP 0 687 594 B1. In the method known from this document, a camera is used to generate an image of a field of view, and the number of black and white pixels in the image is compared to a limiting value. The result of the method is the statement that fog is present when the limiting value is not reached, or that no fog is present when the limiting value is exceeded.
Therefore, this known method only allows a rough estimation. A quantitative measurement of the visual range is not possible, since, during the evaluation of the image generated by camera, no information is present as to which objects the image contains and which contrasts these have by nature. Therefore, the known method can conclude that fog is present, when the field of view is mostly filled in with objects having little contrast.